1. Field of Invention
This invention pertains to a positron emission tomography (PET) detector assembly configuration, or tomograph, for increasing the sensitivity within a subset of azimuthal angles. More particularly, this invention pertains to a PET scanner with a set of detectors configured to detect a greater number of lines of response (LORs) within a first set of azimuth angles than the number of LORs within a second set of azimuth angles. The LORs within the first set of azimuth angles are subject to a greater amount of attenuation than the other LORs because the LORs within the first set of azimuth angles travel a longer distance through the patient and the increased sensitivity compensates for this attenuation.
2. Description of the Related Art
Positron Emission Tomography (PET) is a nuclear imaging technique used in the medical field to assist in the diagnosis of diseases. PET allows the physician to examine the whole patient at once by producing pictures of many functions of the human body unobtainable by other imaging techniques. In this regard, PET displays images of how the body works (physiology or function) instead of simply how it looks. PET is considered the most sensitive, and exhibits the greatest quantification accuracy, of any nuclear medicine imaging instrument available at the present time. Applications requiring this sensitivity and accuracy include those in the fields of oncology, cardiology, and neurology.
In PET, short-lived positron-emitting isotopes, referred to as radiopharmaceuticals, are injected into a patient. When these radioactive drugs are administered to a patient, they distribute within the body according to the physiologic pathways associated with their stable counterparts. As the radiopharmaceutical isotopes decay in the body, they discharge positively charged particles called positrons. Upon discharge, the positrons encounter electrons, and both are annihilated. As a result of each annihilation event, gamma rays are generated in the form of a pair of diametrically opposed photons approximately 180 degrees (angular) apart. After the PET scanner detects these annihilation “event pairs” over a period of time, the isotope distribution in a cross section of the body is reconstructed. These events are mapped within the patient's body, thus allowing for the quantitative measurement of metabolic, biochemical, and functional activity in living tissue. More specifically, PET images (often in conjunction with an assumed physiologic model) are used to evaluate a variety of physiologic parameters such as glucose metabolic rate, cerebral blood flow, tissue viability, oxygen metabolism, and in-vivo brain neuron activity.
Mechanically, a PET scanner consists of a bed, or gurney, and a gantry supporting the tomograph detectors. In some tomographs, the gantry is inside an enclosure having a tunnel through its center, through which the bed traverses. In other tomographs, the detectors are cantilevered over the front of the gantry. In all types of tomographs, the gantry defines a tunnel through which the patient travels. The patient, who has been treated with a radiopharmaceutical, lies on the bed and is moved longitudinally past the detectors. Modern PET tomographs measure the radiation emerging from the patient along a large number of lines of response (LOR), each of which is characterized by four numbers: a radial coordinate, specifying how far the line is from the tomograph's main axis; an azimuthal angle defining the tilt of the LOR in a plane transverse to that axis; an axial coordinate defining the axial position of the LOR's midpoint; and a co-polar angle defining the LOR's tilt angle with respect to the transverse plane. Tomographs can operate either two dimensionally (2D) or three dimensionally (3D). In the 2D case, septa are used to block radiation at large co-polar angles. In the 3D case, no such septa are used. The scanner's sensitivity, and hence its ability to make a good image, is mainly determined by the amount of time that a given body portion of the patient sits before the scanner, and in the 3D case, by the range of co-polar angles, with a larger range corresponding to greater sensitivity. A tomograph's sensitivity is increased by making it relatively longer in the axial direction, so that each body portion of the patient can be scanned for a larger fraction of the total study time. In the case of 3D imaging, the sensitivity is further increased by increasing the scanner length, since this allows the measurement of a greater range of co-polar angles. The combination of the two effects means that the scanner's sensitivity in whole-body imaging scales with the square of its length.
There are four classes of PET tomographs, based on the arrangement of the detectors. Fixed-ring scanners have numerous small detectors organized in detector blocks, which are grouped into buckets, and arranged in an arc around the circumference of the gantry. A second class of PET tomographs includes fixed polygonal arrangements of panel detectors. A third class includes detectors arranged in an arc around the circumference of the gantry, with the detectors rotating about the axis of the gantry. A fourth class includes polygonal arrangements of panel detectors, with the panel detectors rotating about the axis of the gantry.
In each case, the tomograph samples all directions with nearly the same sensitivity. Although it is a simple symmetry for the scanner, this perfect balance is not optimal for whole-body imaging because cross-sections of the body are not symmetrical. That is, the patient's body typically has a greater lateral dimension than an anterior/posterior dimension, when it is imaged with the patient in a horizontal position in the scanner. Accordingly, radiation directed in horizontal and nearly horizontal (lateral) lines of response (LOR's) is strongly attenuated by the patient's body, whereas the radiation in vertical and nearly vertical (anterior/posterior) LOR's is less strongly attenuated. The asymmetry in the measured radiation, combined with the symmetry of the instrument, leads to an undesirable situation in which the data are measured with unequal statistical accuracy. One consequence of this situation is well known to people who read PET images: horizontal streaks across the image in reconstructions done by the filtered backprojection method. These streaks are a common artifact in whole-body PET imaging.
Various articles have been written documenting the processing of data from both ring detector and panel detector scanners. The following articles are representative of the types of scanners and methods of using and processing the LOR data obtained from the detectors. An early reference to the problem of reconstructing images from multi-ring PET scanners is J. G. Colsher, “Fully three-dimensional positron emission tomography,” Phys Med Biol., vol. 25, no. 1, (1980) pp. 103-15. The Colsher paper presents a mathematical algorithm for performing fully three-dimensional positron emission tomography with ring-type scanners. An early description of fully three dimensional (non iterative) reconstruction is by D. Townsend, et al., “Aspects of three dimensional reconstruction for a multi ring positron tomograph,” Eur. J. Nucl. Med., vol. 15, (1989), pp. 741-45. Also, another description is by D. Townsend, et al., “Three Dimensional Reconstruction of PET Data from a Multi-Ring Camera,” IEEE Transactions on Nucl. Sci., vol. 36, no. 1, (February 1989), pp. 1056-65. An early description of polygonally arranged detectors is of the so-called PENN-PET design at the University of Pennsylvania by J. S. Karp, et al., “Continuous-Slice PENN-PET: a Positron Tomograph with Volume Imaging Capability,” J. Nucl. Med., vol. 31, no. 5, (May 1990), pp. 617-27. The Karp paper discusses a scanner with six hexagonally arranged detector panels.